This invention relates to a method for the heat treatment of clay and refractory ware. The invention is particularly concerned with the firing of clay (and refractory) ware containing oxidisable material in a tunnel kiln through which the ware is advanced generally horizontally. The clay may be a heavy clay from which, for example, bricks are formed, or may be a carbonaceous fireclay from which refractories are formed.
Clay from which such products as bricks, tiles, pipes and sanitary ware are formed typically has a moisture content of 2% by weight (excluding chemically combined water of crystallisation). It also typically contains significant proportions of carbonaceous material and sulphur compounds. Its main constituent, however, is aluminium silicate which may take one of a number of forms. It also contains compounds of iron.
The firing of heavy clay ware involves several phases occuring at different temperatures but typically merging into one another. First, residual moisture is driven off at a temperature of up to about 150.degree. C. Loss of water of crystallisation occurs typically between temperatures of 400.degree.-600.degree. C. Volatile organic compounds of carbon are driven off at relatively low temperatures to leave at 600.degree. C. a residue of carbonaceous material that is relatively difficult to remove. Oxidation of this carbonaceous material typically starts in this range and may continue up to or above a temperature of around 900.degree. C. When the clay ware has acquired such temperature, vitrification typically starts to take place and continues until a finishing temperature, typically of around 1100.degree. C., has been reached. The ware is then cooled in the kiln to a temperature in the range 200.degree.-400.degree. C.
A tunnel kiln is designed with the aim of causing all these phases to take place in more or less successive regions of the kiln.
It is important to ensure that all carbonaceous material and sulphur compounds in the clay are expelled or oxidised before the vitrification phase starts. The presence of carbon or sulphur inhibits oxidation of iron compound in the clay. Incomplete oxidation, or "burn-out", of carbon before vitrification can lead to staining of the surface of the ware. Moreover, a deleterious black core is left in the centre of the ware. In extreme circumstances, distortion or bloating of the ware can occur when there is incomplete oxidation of the carbonaceous material.
In practice, difficulties can and do arise in achieving complete oxidation of carbon. These difficulties have lead to a considerable amount of research into the mechanism of oxidation of carbon during the firing of the clay ware. It has been found that the rate of oxidation increases with increasing oxygen concentration at a given temperature. It has further been found that at a given oxygen concentration the rate of oxidation increases with increasing temperature until the temperature at which vitrification takes place is reached. Oxidation will proceed as long as oxygen is available and can diffuse through the surface of the ware. Fusion of the outer layer of the ware during vitrification inhibits oxidation. An analogous problem of burn-out of carbon from heavy clay is associated with the manufacture of refractory ware. It is often necessary to apply an organic binder to the ware before firing in order to give it green strength. A carbon residue is produced within the ware on firing the organic binder. This residue generally needs to be oxidised before vitrification, and the rate of its removal depends on the partial pressure (or concentration) of oxygen in the kiln at the region where oxidation takes place.
Failure to complete oxidation of carbonaceous or organic material in the firing of clay or refractory ware is thus associated with depletion of oxygen in the kiln.
Typically, in operating a tunnel kiln, the ware is set either mechanically or by hand on cars which are passed through the kiln at a chosen rate. This rate, known as the "push-rate" determines the rate of production in the kiln. The ware is stacked on the cars in piles of predetermined dimensions. Often there are two or more piles in each car. Each pile is known as a "setting". The shapes of the settings, and the number of items of ware in each setting, and the spaces for circulation of air between individual items of ware are chosen according to the firing characteristics of the kiln and the properties of the clay or refractory from which the ware is formed. The necessary oxygen is supplied in part by air passed through the kiln counter-concurrently to the ware. The air flow is, however, required for other purposes including the supply of air to help support combustion of fuel passed to the kiln burners, the transfer of heat by convection from the burners to the ware, and the maintenance of a suitable temperature "profile" along the length of the kiln. In addition, it is desirable to keep to a minimum the volume of air passed to the stack of the kiln so as to keep down heat losses. Therefore, it is not generally possible in tackling the problem of obtaining complete oxidation of carbonaceous material to avoid causing other problems if the flow of air through the kiln is increased. Thus, in practice, the operators of tunnel kilns have tended to reach a compromise which, on occasions, entails formation of fired clay or refractory ware containing residual carbon or use of excessive air flows.
Failure to oxidise all the carbonaceous material in the clay arises not so much from a general lack of oxygen in the oxidation region of the kiln as from a local lack of oxygen within a setting at or towards its bottom. Thus, merely to introduce oxygen into the kiln, for example, in the manner described in German Offenlegungsschrift No. 2 754 766, is not in itself a solution to the problem of obtaining adequate oxidation of carbon without expending excessive thermal energy.
There is thus a clear need for an improved method for firing in a tunnel kiln clay or refractory ware containing oxidisable material whereby the oxidation of such material is facilitated without a concomittant requirement for the expenditure of an increased amount of thermal energy.